Refractometry

Refractometers are widely used analytical instruments in quality assurance. To name just a few examples: They measure the refractive index and determining the purity or concentration of ingredients or the sugar content in liquids.
Here you can learn more about the background of this very reliable measuring technique: How does the measurement work and which samples can be measured? How is a refractometer quickly and thoroughly cleaned or calibrated? Does the refractometer fulfil the standard specifications of the measuring task? Which instrument is suitable for the desired application?

Refraktometrie - Alle Proben sind messbar

All samples can be measured

From bright to dark, from basic to acidic, from a low to a high viscosity – our refractometers can measure almost any sample. A high-quality optical system, an intelligent analysis logarithm and a powerful LED allows you to measure extremely challenging samples such as non-homogeneous samples such as fruit juices with pulp, opaque samples such as heavy oils or emulsions such as cosmetics. Aggressive acids and bases can be easily measured using a chemical resistant stainless steel measuring tray and a sapphire prism. Even foils and solids can be measured by using a high-refractive contact liquid.

Einflussfaktor Temperatur

Influencing factor temperature

If the temperature of a sample varies by 1 °C, this will then become already apparent with the fourth decimal place of the measured value. A precise temperature control is therefore extremely important in order to ensure a high accuracy and reproducibility of the measurement results. This is realised either through a built-in Peltier element or through external thermostats. This allows for standard-compliant measurements at various temperatures, for example, at 20 °C (Ph. Eur) or at 25 °C (USP). A quick temperature control should be ensured; this will save a lot of time in case many temperature changes take place.

Peltier-Temperierung

Peltier temperature control

The built-in Peltier element makes it possible to control the temperature of samples within a range of 10–80 °C in order to be decoupled from external temperature influences. The selected temperature has a very high accuracy of up to 0.01 °C ensuring the comparability and reproducibility of the measurement results. An extremely homogeneous temperature control also rules out any measurement deviations due to temperature gradients. In addition, our temperature control offers short heating and cooling times; temperature changes between 20 °C and 70 °C are achieved in less than four minutes.

vTemperaturkompensation

Temperature compensation

A temperature control of the measurement sample is not required when sugary beverages and confectionary products are measured. A conversion table published by the ICUMSA, which shows the influence of temperature on the refractive index of sucrose, glucose, fructose and invert sugar solutions, makes this possible. Since the influence of the temperature on the measured value is known, it is possible to take measurements at any ambient temperature and to automatically convert them to the desired reference temperature – often 20 °C. Temperature differences will also be automatically compensated.

Kalibrieren und Justieren

Calibration and adjustment

The refractometer should be inspected on a regular basis to ensure that it delivers reliable measurement results. An easier method is the testing of the water value. The refractive index (nD) of distilled water at 20 °C (589 nm) is exactly 1.33299. If this value is not met, you can use the tare function to carry out a one-point adjustment; the device adjustment is compared with the currently measured value. Certified, traceable standards, which we also use for every commissioning, IQ/OQ/PQ or annual maintenance work, are recommended for a subsequent calibration in order to validate the specified measurement accuracies of our devices.

Scales

Scales

There is a relationship between many physical measured variables. If these dependent relationships are known, you will only have to measure one variable from which all the others can be calculated. In the field of measurement technology we refer in terms of scales, for example, when the refractive index is converted into a suitable substance-specific unit. Of over 100 commonly used scales, the most frequently used is the BRIX scale. It is used for determining the sugar concentration in aqueous solutions and is often pre-installed on refractometers. This scale (1%BRIX corresponds to a 1% sucrose/water solution) is used widely, e.g. for content determination in the chemical or automotive industry. If standard scales are no longer sufficient, the formula editor in our digital refractometers helps. It can convert individualised mathematical relationships between concentration and nD value into a formula.

What is Refractometry?

When we view objects through the surface of water, how does the refraction of light result in us seeing such objects with a “kink“?
What does the “kink” have to do with the refractive index and refractometers?
How do refractometers measure the purity and concentration of ingredients or the sugar content of beverages?

What is Refractometry?

Standards and directives

Refractometers are designed and used in accordance with standards. Whoever uses refractometers according to normative specifications can guarantee that measurements are correct and work according to reproducible procedures. We have compiled an overview of standards and guidelines that refer to refractometers.

Refractometers and standards

Samples and measurement values

With the right refractometer it is possible to measure a large variety of samples in a wide range of applications. To select the right refractometer, it is helpful to define the required measuring range. Find out more: We have put together a comprehensive overview of measurement samples with the specific refractive index.

Samples and measurement values

Refactometer applications

Refractometers are used for quality assurance in the pharmaceutical, chemical, food and beverage industries as well as in the production of flavours, perfumes and essential oils or in the petrochemical, metalwork and automotive industries. Generally speaking, the determination of the refractive index helps to examine the purity of a substance quickly and reliably. In addition, the progress of reactions and substance conversions can be read via the refractive index.

Typical applications

Cleaning tips for your refractometer

Basic requirement for precise measurement results is the exact cleaning of the refractometer. The correct cleaning does not have to be time-consuming. We have achieved good cleaning results with the following cleaning agents and procedures.

Cleaning tips

Automation with refractometers

The DR6000-TF models with through-flow function, flow-through cell, drying unit DS7060, peristaltic pump DS7070 and autosampler AS80 or AS90 allow for semi- or fully automated operation.

more about automation

Any Questions?

We will be glad to help.

A.KRÜSS Optronic GmbH
Alsterdorfer Str. 276-278
22297 Hamburg | Germany

Tel. +49 40 5143 17-0
Fax +49 40 5143 17-60

E-Mail sales-labor@kruess.com